14:00 〜 14:15
[AOS09-02] Background Factors for the Interannual Variations of the Realization of the Madden–Julian Oscillation
The Madden–Julian oscillation (MJO) is an eastward-propagating large-scale convective system coupled with zonal overturning circulations on the planetary scale in the tropics. In canonical views, MJO convection is realized over the Indo-Pacific warm pool quasi-periodically on the intraseasonal time scale (20–100-day period), which is the most prominent in boreal winters. Meanwhile, it is also known that the year-to-year variability of the longevity of boreal-winter MJOs is clearly noticeable; MJO realization is active in some years and inactive in other years. Since whether the MJO is vigorously realized or not is one of important aspects affecting the activities of other weather and climate phenomena, it is worthwhile to reveal what can determine active and inactive years of MJO realization and utilize the revealed mechanisms for the sub-seasonal prediction.
This study is aimed at understanding the background conditions and mechanisms that control the interannual variations of the activity of MJO realization in boreal winters, using the observation-based data in 1979–2018. The activity of MJO realization in each boreal winter (November to March) from 1980/81 to 2017/18 is measured by days on which MJO events detected by the tracking of the Wheeler-Hendon index are active. Results suggest that, in active years of MJO realization, boreal-winter mean convection tends to be enhanced over the western Pacific (WP) and suppressed over the Maritime Continent, and vice versa in inactive years. This contrast can result from collective (i.e., not separable) effects of the El Niño–Southern Oscillation (ENSO) and quasi-biennial oscillation (QBO); MJO realization is prompted under the moderate El Niño conditions irrespective of QBO phases, and in other ENSO phases, the more enhanced easterly phases of the QBO favor MJO realization. For the latter, which is almost unrelated to the ENSO amplitudes, the zonally inhomogeneous QBO response that the static stability near the tropopause associated with QBO easterlies is more decreased in the WP may support enhanced background convection there.
In comparison of individual MJOs in the MJO active and inactive years, the number of MJOs initiated in the Indian Ocean is indeed much smaller in inactive years, whereas MJO amplitudes are not so different from each other. In addition, MJOs in inactive years tend to fail to propagate into the WP because of the disruption of vertical advective moistening, which is affected by the background suppressed convection in the WP. To sum up, the cooperation of the ENSO and QBO changes background convection and circulations over the warm pool, which in turn largely modulates the frequency of MJO initiation and the duration of MJOs.
This study is aimed at understanding the background conditions and mechanisms that control the interannual variations of the activity of MJO realization in boreal winters, using the observation-based data in 1979–2018. The activity of MJO realization in each boreal winter (November to March) from 1980/81 to 2017/18 is measured by days on which MJO events detected by the tracking of the Wheeler-Hendon index are active. Results suggest that, in active years of MJO realization, boreal-winter mean convection tends to be enhanced over the western Pacific (WP) and suppressed over the Maritime Continent, and vice versa in inactive years. This contrast can result from collective (i.e., not separable) effects of the El Niño–Southern Oscillation (ENSO) and quasi-biennial oscillation (QBO); MJO realization is prompted under the moderate El Niño conditions irrespective of QBO phases, and in other ENSO phases, the more enhanced easterly phases of the QBO favor MJO realization. For the latter, which is almost unrelated to the ENSO amplitudes, the zonally inhomogeneous QBO response that the static stability near the tropopause associated with QBO easterlies is more decreased in the WP may support enhanced background convection there.
In comparison of individual MJOs in the MJO active and inactive years, the number of MJOs initiated in the Indian Ocean is indeed much smaller in inactive years, whereas MJO amplitudes are not so different from each other. In addition, MJOs in inactive years tend to fail to propagate into the WP because of the disruption of vertical advective moistening, which is affected by the background suppressed convection in the WP. To sum up, the cooperation of the ENSO and QBO changes background convection and circulations over the warm pool, which in turn largely modulates the frequency of MJO initiation and the duration of MJOs.